TAILCOAT

A 'tailcoat', sometimes a 'swallow-tail' coat is the coat traditionally worn by men for white tie dress, for very formal evening occasions.
A tailcoat is waist length in the front and sides, and has two long tails reaching to the knees in back. (Sometimes with a pocket on the inside meant to hold gloves). In this respect the tailcoat is a modification of the older frock coat: the frock coat goes down to the knees all the way around, and lower edge of its other descendant, the morning coat or cutaway, slants diagonally from the waist in front to the knees in back, the tailcoat is cut away more dramatically across.
The tailcoat traditionally also has satin facings on the lapels, is double breasted, and meets but does not fasten in the front. (There are two rows of buttons, all non-functional.)
It is normally worn with a white wing-collar dress shirt with single cuffs fastened with cufflinks, a matching white bowtie and waistcoat, black trousers, and black patent leather shoes with leggings.
Although regarded as formal wear today, like all such "formal wear" it is based upon standard fashions of earlier times. Tailcoats, sometimes called a "claw-hammer", would be standard day-to-day wear in the 1830's to 1850's. A period photo of President James Buchanan and his cabinet shows a group of men all wearing this type of garment.